A variety of studies are going ahead on the plate-making technique for computer-to-plate system which has progressed increasingly in recent years. Of such techniques, as those aiming at further rationalization of processes and the solution of the problem of waste solution treatment, lithographic printing plate precursors which can be mounted on a printing machine immediately after exposure without undergoing development process and can be used for printing have been researched and various methods are suggested.
As one method of doing away with a treating process, there is a method called on-press development comprising the steps of loading an exposed printing plate precursor onto the plate cylinder of a printing machine and supplying a fountain solution and an ink with rotating the plate cylinder to thereby remove the non-image domain of the coated layer of the printing plate precursor. That is, this is a method of exposing a lithographic printing plate precursor, loading it immediately onto a printing machine and effecting developing treatment in a general printing process.
Such a lithographic printing plate precursor suitable for on-press development is required to have a photosensitive layer soluble in a fountain solution or an ink solvent and have daylight handleability capable of undergoing development on a printing machine in a bright room.
For example, a lithographic printing plate precursor comprising a hydrophilic support having provided thereon a photosensitive layer comprising a hydrophilic binder polymer having dispersed therein fine particles of a thermoplastic hydrophobic polymer is disclosed in Japanese Patent 2938397. This is disclosed in this patent to coalesce thermoplastic hydrophobic polymer fine particles by the heat of infrared laser exposure to form an image on a lithographic printing plate precursor, and to mount the printing plate precursor on the cylinder of a printing machine to perform on-press development with a fountain solution and/or an ink. Due to the sensitive region being infrared rays, the printing plate precursor is handleable in daylight.
Further, the techniques of coalescing thermoplastic fine particles by heat and making a printing plate by on-press development are also disclosed in JP-A-9-127683 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and WO 99/10186.
However, these methods of making an image by the coalescence of fine particles by heat are accompanied by the problem that high press life (i.e., high printing durability) is difficult to obtain, although good on-press developability can be obtained.